Chaos seems to aid learning

By
Kimberly Patch,
Technology Research NewsAlthough it's clear that the cerebellum
is the part of the human brain involved in coordinating movements in ways
that allow people to learn skills like riding a bike, there are mysteries
about how the learning process works.

Researchers from Core Research for Evolutional Science and Technology
(CREST) in Japan have built a computer simulation of the inferior olive,
a portion of the brain that probably relays errors in movement to the
cerebellum. It has been difficult to explain the mechanics of this relationship
because inferior olive cells that connect to the cerebellum fire slowly,
and this does not fit well with the common hypothesis that high-fidelity
error signals are needed for efficient learning.

The researchers got the idea for the simulation after initial
research showed that if neurons were electrically coupled, or linked,
a certain type of chaotic signal could emerge.

The researchers' simulation shows that moderate electrical coupling
between nerve cells in the inferior olive could produce a type of chaotic
firing that effectively recodes the high-frequency information into slower
signals by imparting information within the rhythm rather than just the
frequency of nerve firing. "The chaotic firing was more robust than we
expected," said Nicholas Schweighofer, a researcher at Core Research for
Evolutional Science and Technology. The model shows that "chaos can be
useful in the brain," he said.

In addition to allowing researchers to better understand the mechanics
of the brain, the researchers' theory of chaotic resonance could speed
electronic communications and improve robotics. "In communications, our
work [could] maximize the information transmitted in networks," he said.
"In robotics, chaos could be used to explore the environment to optimize
learning," he said.

Electrical signals carry information from one end of a nerve cell
to the other, while a chemical reaction is responsible for passing signals
from one cell to another through their interconnected dendrites, or nerve
cell fibers.

The researchers' results explain some unusual properties of the
inferior olive cell input to the Purkinje cells of the cerebral cortex,
according to Schweighofer. Each Purkinje cell contains two types of nerve
synapse inputs, or connections to other nerve cells. The cells connect
to about 100,000 other nerve cells via parallel connections, but have
only a single connection to an inferior olive neuron.

The parallel connections generate simple nerve spikes, or on signals,
but the inferior olive connection generates a more complicated signal.
Experiments have also uncovered apparently random firing, and chaotic
subthreshold activity, or signals that are not strong enough to trip the
chemical reaction that ordinarily passes a signal to neighboring cells.
It is also known that the inferior olive neurons are electrically coupled.

It was a challenge to make a realistic model of the inferior olive,
said Schweighofer. "Finally showing the existence of chaos... necessitated
very lengthy computations," he said.

The researchers' inferior olive cell models included the known
location of the ionic currents that carry signals between nerve cells,
the gap junctions between the cells and the synaptic inputs.

The researchers modeled two types of networks of a few simulated
inferior olive cells: chain networks, and grid networks. In chain networks,
each neuron is electrically coupled to its one or two neighboring cells
depending on its position in the chain. In grid networks of 2 by 2, 3
by 3, and 9 by 3 cells, cells are connected to two, three or four neighbors
depending on their grid positions.

When the researchers removed to the connections between cells,
each cell generated plain periodic spikes, or signals at an average rate
of 3.1 spikes per second. When the researchers connected cells within
a network using just an intermediate coupling strength, the firing pattern
of individual cells appeared chaotic and the average firing rate was reduced
to 1.8 spikes per second. When the researchers used a strong coupling
strength, the cells generated regular, synchronized spikes at a firing
rate of 3.5 spikes per second. These results are consistent with experiments
on actual nerve cells.

The simulation showed that moderate electrical coupling speeds
information transfer, according to Schweighofer. The mutual information
per spike for a single cell at the center of the 3 by 3 networks was 48
percent greater than the same network without coupling and 37 percent
greater for the coupled network as a whole despite the lower spike-per-second
rate. The researchers found similar results for the other types of networks.

Now that they have proved computationally that chaotic signals
are capable of carrying extra information, the researchers are aiming
to show empirically that this is what happens. The next step is "doing
in vivo work showing that chaos actually exists in the inferior olive,"
said Schweighofer.

Schweighofer's research colleagues were Kenji Doya, Hidekazu Fukai,
Jean Vianney Chiron, Tetsuya Furukawa and Mitsuo Kawato. The work appeared
in the March 30, 2004 issue of Proceedings of the National Academy of
Sciences. The research was funded by the Telecommunications Advancement
Organization and the Human Frontier Science Program.